Generally, UOE metal pipes such as UOE steel pipes for line pipes are laid after being joined to each other by peripheral welding at the laying site. In performing the peripheral welding, the groove faces formed in the pipe end portions of the UOE metal pipes must be butted against each other with high accuracy. If the butting accuracy is inadequate, peripheral welding quality may be degraded and efficiency may be lowered due to the increase in man-hours for correction and, in the worst case, the UOE metal pipes cannot be joined to each other. In order to increase the butting accuracy of groove faces, the roundness in the pipe end portion must be improved, and tight roundness specifications are imposed on the UOE metal pipes.
Usually, the roundness of a UOE metal pipe is improved by expanding the pipe. However, since the principal objective of the pipe expanding process is to wholly correct a difference between the longitudinal diameter and the transverse diameter of the UOE metal pipe, the fulfillment of requirement for local roundness is limited. In order to improve the roundness, a portion where the shape is difficult to secure, such as surroundings of weld bead, must be corrected locally. Especially in the surroundings of weld bead, a remaining chevron-shaped portion of what is called peaking formed at the time of pressing causes the shape of UOE metal pipe to deviate from the ideal truly round shape. For the UOE metal pipe, the peaking formed in the surroundings of weld bead has been a main cause of hindering the improvement in roundness.
Accordingly, a shape correcting apparatus for improving the roundness of the pipe end of a UOE metal pipe for line pipe has conventionally been proposed. For example, in the correcting machine described in Patent Document 1, the roundness of a steel pipe is corrected by using a pair of circular arc-shaped dies (an outer surface-side die and an inner surface-side die). Specifically, a pressure is applied to the steel pipe by the outer surface-side die and the inner surface-side die while the outer surface-side die is in contact with the outer peripheral surface of steel pipe and the inner surface-side die is in contact with the inner peripheral surface of steel pipe, whereby the roundness of steel pipe can be corrected.